Methods and apparatus for applying wear resistant coatings to rotogravure cylinders

ABSTRACT

A coating of wear resistant tungsten carbide is applied to a photo-etched roto-gravure cylinder, the coating having a thickness in the range of from 15 to 35 microns. Tungsten carbide powder is supplied to a plasma flame spray gun by means of a powder feeder which supplies powder particles at a uniform density and at given sizes. The feeder comprises a hopper having air inlet and outlet passages and vertically spaced porous membranes which confine the powder and permit the passage of air. Air flow through the hopper suspends the powder particles of a desired diameter range adjacent a spray gun feed conduit. When copper cylinders are to be coated, the surface is allowed to oxidize to promote a chemical bonding of the coating to the surface.

BACKGROUND AND OBJECTS OF THE INVENTION

The present invention relates to the application of tungsten carbidecoatings to photo-engraved roto-gravure cylinders, especiallyroto-gravure printing cylinders.

Roto-gravure cylinders are commonly employed as metering or anilox rollsto transfer uniform coatings of ink or the like, and as printing rollsto transfer a specific ink pattern. Such cylinders are generallyfabricated by a photo-engraving process wherein a cellular surface isproduced on the cylinder periphery which functions to receive ink from asource, and transfer it to a web or another roll.

Since roto-gravure rolls are subject to rapid wear, and to the corrosiveeffects of printing inks, it has been common to plate such rolls withsubstances such as copper or chrome to maximize durability. Plating withcopper and chrome is very expensive and does not provide as muchdurability as a tungsten carbide coating. Tungsten carbide coatings havebeen applied with some success to mechanically engraved metering rollsby a flame spray technique, but such success has not been achieved inconnection with roto-gravure printing rolls. In that regard, thepresently utilized flame spray techniques result in the application of acoating which is excessively thick for roto-gravure printing rolls andthus diminishes the definition of the cell pattern.

For example, the disclosure of a tungsten carbide coating of 0.002 to0.008 inches on a metering roll in U.S. Pat. No. 4,009,658 would beexcessively thick for a gravure printing roll. Although the cells arenot filled to such an extent as to unduly impair the performance of agravure metering roll, a gravure printing roll coated to this thicknesscannot perform at acceptably high levels of quality in most instances.Accordingly, gravure printing rolls continue to be coated with morecostly and less durable copper and substances, such as chrome, which canbe applied in sufficiently thin coatings.

When applying coatings of substances such as chrome to copperroto-gravure printing cylinders, grit-blasting step has been performedto form a pitted surface on the copper surface to facilitate adherencethereto of the coating, the latter entering the surface pits to create amechanical bond between the coating and the cylinder. Prior to theapplication of the coating, the copper surface is thoroughly cleaned andspecial precautions are taken to prevent oxidation of the copper surfacesince the formation of a copper oxide film has been herefore consideredto be detrimental. It would be desirable to eliminate the need for thegrit-blasting step which adds appreciably to the fabrication costs ofthe cylinder.

It is therefore, an object of the present investigation to provide novelmethods and apparatus for coating roto-gravure cylinders.

It is another object of the invention to enable tungsten carbide to beapplied to roto-gravure printing cylinders, especially roto-gravureprinting cylinders, in a relatively thin layer.

It is a further object of the invention to enable a tungsten carbidecoating to be flame sprayed onto a roto-gravure cylinder at a thicknessin the range of from 15 to 35 microns.

It is an additional object of the present invention to eliminate theneed for a grit-blasting step for the application of wear-resistantcoatings to copper roto-gravure cylinders and rolls.

It is yet another object of the invention to create a chemical bondingbetween coatings, such as of tungsten carbide, and copper roto-gravurecylinders and rolls.

SUMMARY OF THE INVENTION

These objects are achieved by the present invention which involvesfeeding powder to a plasma flame spray gun for applying a coating to aroto-gravure cylinder. The apparatus comprises a powder hopper definingan internal chamber. The hopper includes a gas inlet at a lower endthereof for introducing pressurized gas into the chamber, and a gasoutlet at an upper end of the chamber for exhausting gas from thechamber. A lower porous membrane is disposed across the chamber abovethe gas inlet and an upper porous membrane is disposed across thechamber above the lower membrane and below the gas outlet. The upper andlower porous membranes are pervious to the gas and impervious to thepowder. A powder discharge opening communicates with the chamber betweenthe porous membranes for conducting powder to the spray gun. Pressurizedgas is supplied to the gas inlet for suspending powder particles withinthe chamber between the upper and lower membranes such that particles ofa selected size range are suspended at a substantially uniform densityadjacent the powder discharge opening and are discharged therethrough.

Preferably, tungsten carbide particles are fed from the hopper todeposit a coating of tungsten carbide on the cylinder in the range offrom 15 to 35 microns thickness.

The invention is particularly advantageous in connection with thecoating of photo-etched roto gravure printing cylinders because the thincoating does not unduly impair the printing performance.

Another aspect of the invention involves a method for coating a coppersurface of a roto-gravure cylinder with a wear resistant substancecomprising the steps of cleaning the copper surface, forming a film ofcopper oxide on essentially the entire copper surface, and spraying thewear-resistant substance in molten form from a plasma flame spray gunonto the copper oxide film to form the coating. By forming a copperoxide film, a chemical bonding of the coating is achieved, whicheliminates a grit blasting step which is needed in cases where amechanical bonding is achieved.

THE DRAWING

These and other objects and features of the invention will becomeapparent from the claims and from the following description when read inconjunction with the accompanying drawings.

FIG. 1 is a schematic view of a system for applying a tungsten carbidecoating to a roto gravure roll according to the present invention;

FIG. 2 is a vertical cross-sectional view through a powder feedingmechanism of the system; and

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

A system 10 is depicted in FIG. 1 for applying a coating of tungstencarbide to a photo-etched rotor-gravure roll 12. The coating systemincludes a plasma flame spray gun 14 of a conventional type which ismounted on a movable carrier 16. An electric supply conduit 15 and anargon gas supply conduit 17 extend within the carrier and feed into theflame gun. Powder supply hoses 18 communicate a feed mechanism 20 withthe flame gun for conducting a powder, preferably tungsten carbide, tothe gun. The spray gun itself is well known and further details thereofare not needed. Suffice it to say that an electric arc is established bythe gun which melts tungsten carbide powder particles that areintroduced into the gun. The melt is then sprayed from the gun by theargon gas.

As is conventional, the gravure roll 12 is mounted for rotation aboutits longitudinal axis and the gun carrier 16 is mounted fortranslational motion in a direction parallel to the roller axis. Therotational speed of the roller and the translational speed of the gunare correlated so that the entire periphery of the roller is coated.

As noted earlier, in order for the cells of a gravure printing rollcoated with tungsten carbide to print with required definition, thecoating must be applied thin enough to avoid an excessive fill-in of thegravure cells of the roll. For example, an acceptable coating is oneapplied at a thickness in the range of from 15 to 35 microns, dependingupon the cell size of the particular gravure roll being treated. In thisregard, it is preferable that the selected thickness be controlledwithin a tolerance of 2.5 microns.

In order to achieve this goal with the flame spray process,it isvirtually essential that the density of the sprayed tungsten carbidepowder particles remain uniform and constant, and that the size of thesprayed powder particles lie within a given range. Substantialdifficulties have been heretofore encountered in achieving theserequirements. In this regard, it is noted that commercially availablebatches of tungsten carbide powders contain particles of an excessivelywide range of diameters.

Accordingly, the powder feed mechanism 20 of the present invention hasbeen provided which enables a gravure printing roll to be coated with asufficiently thin layer of tungsten carbide, i.e., within a range of 15to 35 microns.

The powder feed mechanism comprises a hopper 22 having a cylindricalupper portion 24 of circular cross section and a conical lower portion26 of circular cross section defining an internal chamber 28. Anentrance 30 for a gas, such as air, is provided at a lower end of theconical portion 26, and an air outlet 32 is provided in a cover 34 whichcovers the upper portion 24. An air delivery duct 36 (FIG. 1) supplies astream of pressurized air which enters the chamber 28 through the inlet30 and exits the chamber through the outlet 32.

Extending completely across the cross section of the conical portion 26above the air inlet 30 is a lower circular porous membrane 38, andextending completely across the cross section of the cylindrical portion24 below the air outlet 32 is an upper circular porous membrane 40. Themembranes 38, 40 are conventional and comprise microporous polyethylenewith a mesh size in the range of 5 to 10 microns, which is sufficient toconduct gas flow but prevent the passage of powder particles. A membranethickness of 3/8" is preferred.

Disposed in the wall of the cylindrical portion 24 is a closable loadinghatch 42 through which tungsten carbide powder can be introduced intothe chamber 28.

Before a feeding operation commences, powder within the hopper 22 restsupon the lower membrane 38.

A series of powder discharge tubes 44, preferably four in number, arepositioned equidistantly around the cylindrical portion 24 andcommunicate with the plasma gun 14 by means of the flexible hoses 18.The discharge tubes 44 lie in a horizontal plane located at a levelapproximately midway along the height of the cylindrical portion 24. Theinlet ends 46 of the tubes 44 are positioned approximately midwaybetween the central vertical axis of the hopper and the wall 48 of thecylindrical portion 24.

By passing air through the chamber 28, the tungsten carbide powdertherewithin becomes suspended and dispersed within the chamber.

At a given constant air velocity, powder particles of similar volumetend to remain suspended at a particular level and density within thechamber. The discharge tubes 44 are positioned at a level correspondingto a desired particle size and density. By varying the air velocity,those characteristics of the suspended powder can be controlled.

The air velocity and pressure are controlled by regulators 50, 52 in theair inlet and outlet lines. In this fashion, it is possible to regulatethe discharge velocity of the particles through the discharge tubes 44.For example, it is preferable to maintain a pressure of 20-25 psithrough the chamber 28 when feeding tungsten carbide particles.

In order to prevent the upper and lower membranes 40, 38 from becomingclogged with powder, a pair of upper and lower rotary cleaner brushes52, 54 are employed to "scrub" the powder-contacting surfaces of thosemembranes. The brushes 52, 54 each comprise a plurality of radial spokes56, 58 to which carry bristles 60, 62. The spokes 56, 58 are connectedto a common drive shaft 64 extending vertically axially through thehopper 22. A drive motor 66 is attached to the shaft to drive thebrushes 52, 54 at selected speeds. The bristles 60, 62 are positioned toengage the membranes and scrub powder particles therefrom. This assuresthat the lower membrane will be unobstructed and available to pass airto the powder, and that a uniform density level of powder will bepresent adjacent the lower membrane for contact with the air stream.

During the travel of air through the chamber 28, a swirling air streamis established due to the conical configuration of the lower portion 26of the hopper. Such swirling action creates a random flow pattern of thepowder particles, assuring that a uniform particle density (i.e.,particle count per volume) is established within the chamber 28.

Heavier powder particles may tend to gravitate downwardly along thesides of the hopper. Such particles are caused to approach the center ofthe hopper, due to the inwardly sloping nature of the wall of theconical portion 26. Those particles are maintained in a fluidized cloudabove the lower membrane and, gradually gravitate downwardly onto themembrane 38, whereupon they are immediately propelled upwardly by theair flow through the lower membrane. The lower brush 54 stirs theparticles and prevents undue clogging of the membrane 38.

The height to which the particles may rise in the chamber, for a givenair pressure, is a function of the size of the particles and the airvelocity within the chamber. That is, heavier particles will not belifted as high as the lighter particles per a given air velocity. It hasbeen found that for a given air pressure through the chamber, the powderparticles will be suspended at a substantially uniform density, withparticles of generally common size being situated at a respective levelin the chamber. Thus, by suitable regulation of air pressure through thechamber, particles of a desired size can be discharged through thedischarge tubes 44. Accordingly, it will be appreciated that the feedermechanism functions to classify particles according to size.

Moreover, the heavier, unwanted particles will remain suspended at thelower end of the chamber and may eventually be disposed of when thecoating operation is finished.

A plurality of heating elements 70 are positioned around the exterior ofthe cylindrical portion 24 to maintain the chamber at a temperaturesufficient to eliminate condensation which could otherwise be absorbedby the powder. It has been found that such a goal can be achieved bymaintaining the chamber at about 150° F.

It will be appreciated that the particle feeding mechanism is adapted toapplying a tungsten carbide coating or the like to all types ofroto-gravure cylinders.

In the fabrication of a copper roto-gravure printing cylinder 12, thecylinder is initially photo-etched in a conventional manner. Thereafter,the cylinder 12 is mounted in a rotational machine and is thoroughlycleaned with trichloroethylene (C₂ HCL₃) to remove soil that may bepresent on the surface. The cylinder is then rotated and pre-heated,using the moving plasma torch as a heat source, until a desiredtemperature of the cylinder, preferably from 110°-120° F., is obtained.The preheating cycle removes moisture that might be entrapped in themicroporous surfaces of the engraved areas on the cylinder. The cylinder12 is then cleaned again with trichloroethylene to remove any oils thatmay have exuded during the pre-heating cycle.

As noted earlier, care has heretofore been taken to prevent theoccurrence of appreciable oxidation of the copper surface becauseoxidation was believed to be detrimental to the application of thecoating. However, it has now been discovered that by promotingoxidation, a chemical bond can be established between the copper surfaceand the tungsten carbide coating.

Accordingly, oxidation is promoted by exposing the copper surface to airfor a selected period, e.g., about fifteen minutes, or by heating thesurface by a flame torch (with the powder feeder 20 maintainedinactive). Accordingly, a film of copper oxide is established onessentially the entire cylindrical surface of the cylinder, resulting ina chemical bond being created with the subsequently-applied tungstencarbide coating. While the inventor does not wish to be held to aspecific theory, it is surmised that a crystal lattice structure isformed between the copper surface and the tungsten carbide coating tocreate such mechanical bond.

The copper oxide film which is established on the copper surface can becupric oxide, cuprous oxide or a combination of both.

The chemical bond which is created is significant in that it eliminatesthe mechanical grit-blasting operation which has heretofore beenperformed in the conventional mechanical bonding of the coating to thecylinder. The chemical bonding according to the present invention ismore adherent and there is no distortion of the cell structure as canresult from grit-blasting.

It is possible that, heretofore, during performance of the prior artcoating techniques, a slight amount of oxidation may have incidentallyoccurred on the copper cylinder surface prior to application of thewear-resistant coating, despite efforts taken to prevent same. However,if essentially the entire copper surface has been oxidized, the coatingwould not be applied. Rather, the surface would be re-cleaned beforecommencement of the coating step.

After being formed, the oxidized film is preheated with the flame torchpreferably to a temperature in the range of from 180° F. to 220° F.Preheating is accomplished, as before, by means of the flame torch 14with the cylinder rotating and the powder feeder inactive.

After the preheating step, the torch 14 is returned to a startingposition and fixed at the proper distance from the face of the cylinderfor a coating step. The powder feeder is activated by forcing airthrough the air inlet 30 of the powder-containing chamber 28. Airpassing through the container entrains the powder particles and suspendsthem at a substantially uniform density within the chamber, withparticles of common sizes becoming situated at respective levels withinthe chamber. The inlet and outlet pressure valves are to achieve adesired air velocity and powder density within the chamber to assurethat particles of a diameter in the range of from 1 to 8 microns aredisposed at the level of the powder discharge tubes 44. The valves arealso adjusted to achieve a desired powder velocity through the powderdischarge tubes 44. The brushes 38, 40 are rotated to eliminate powderbuild-up on the upper and lower membranes.

The cylinder 12 is then rotated at the desired speed and the torch 14 istranslated at the desired speed as a plasma stream of tungsten carbideis emitted from the torch.

Powder particles in the range of from 1 to 8 microns are supplied at auniform density to the torch, enabling a coating to be applied which isof uniform thickness in the range of from 15 to 35 microns, a coatingwhich was heretofore not possible to achieve with consistent uniformity.After the completion of the coating operation, the cylinder istransferred to a wet polishing machine.

The end result is a gravure printing cylinder having a much higherresistance to abrasion and erosion, and a life expectancy nearly tentimes that of a conventional chromium plated cylinder.

The thin, uniform coating made possible by the present invention isideally suited to roto-gravure printing cylinders because the definitionof the printing cells is not destroyed. It will be appreciated, ofcourse, that the present invention is also applicable to the coating ofroto-gravure metering or anilox cylinders with tungsten carbide.

The creation of a copper oxide film on the copper surface of aroto-gravure rolls prior to the coating of tungsten carbide, enables thecoating to be bonded chemically, thereby eliminating the conventionalgrit blasting step.

For principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, sincethese are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art withoutdeparting from the spirit of the invention.

What is claimed is:
 1. Apparatus for feeding powder to a plasma flamespray gun for applying a coating to a roto-gravure cylinder, saidapparatus comprising:a powder hopper defining an internal chamber whichincludes a lower conical portion and an upper cylindrical portion, saidhopper including a gas inlet at a lower end thereof for introducingpressurized gas into said chamber, and a gas outlet at an upper end ofsaid chamber for exhausting gas from said chamber, a lower porousmembrane disposed across said chamber above said gas inlet and below atleast part of said conical portion such that gas passing through saidlower membrane enters said conical portion and is caused to swirl, anupper porous membrane disposed across said chamber above said lowermembrane and below said gas outlet, said upper and lower porousmembranes being pervious to said gas and impervious to said powder,powder discharge opening means opening into said cylindrical portion ofsaid chamber between upper and lower ends of said cylindrical portionand between said porous membranes, said discharge opening means arrangedto conduct powder to the spray gun, and means for supplying pressurizedgas to said gas inlet for suspending powder particles within saidchamber between said upper and lower membranes such that suspendedparticles of a selected size range are suspended at a substantiallyuniform density adjacent said powder discharge opening means and aredischarged therethrough.
 2. Apparatus according to claim 1, wherein saiddischarge opening means faces substantially radially.
 3. Apparatusaccording to claim 1 wherein said powder discharge opening means aredisposed in said cylindrical portion.
 4. Apparatus according to claim 3wherein said powder discharge opening means comprise a plurality oftubes extending into said chamber.
 5. Apparatus according to claim 4wherein the inlet ends of said tubes extend radially approximatelymidway between a wall of said cylindrical portion and a vertical centralaxis of said hopper.
 6. Apparatus according to claim 1 including drivenbrush means for brushing powder from the lower surface of said uppermembrane and the upper surface of said lower membrane.
 7. Apparatusaccording to claim 6 wherein said brush means comprise a pair of brushesmounted on a common vertical drive shaft, such brush including ahorizontal bristle holder and bristles mounted therein.
 8. Apparatusaccording to claim 1 including gas pressure regulating means forcontrolling gas pressure at said inlet and gas pressure at said outlet.9. Apparatus according to claim 1 including heating means for heatingsaid chamber.
 10. Apparatus for applying a coating of tungsten carbideat a thickness of from 15 to 35 microns to a roto-gravure cylinder,comprising:a plasma flame spray gun for emitting a spray of moltentungsten carbide, and means for feeding tungsten carbide particles tosaid spray gun at a substantially uniform density, said feeding meanscomprising:a hopper comprising a lower conically shaped portion and anupper cylindrically shaped portion, said portions defining an innerchamber of said hopper for containing tungsten carbide powder, saidhopper including a gas inlet at a lower end of said conical portion anda gas outlet at an upper end of said cylindrical portion, a lower porousmembrane disposed across said conical portion above said gas inlet andbelow at least part of said conical portion such that gas passingthrough said lower membrane enters said conical portion and is caused toswirl, an upper porous membrane disposed across said cylindrical portionbelow said air outlet,said upper and lower membranes being pervious tosaid gas and impervious to said powder, powder discharge opening meansin said cylindrical portion opening into said cylindrical portion ofsaid chamber between upper and lower ends of said cylindrical portionand between said membranes, said powder discharge opening means beingconnected to said spray gun, and means for supplying pressurized gas tosaid gas inlet for suspending said powder within said chamber betweensaid upper and lower membranes such that suspended particles of 1 to 8microns in diameter are suspended at a substantially uniform densityadjacent said powder discharge opening means and are dischargedtherethrough.
 11. Apparatus according to claim 10 wherein said powderdischarge opening means comprise a plurality of tubes extending intosaid chamber.
 12. Apparatus according to claim 11 wherein the inlet endsof said tubes extend radially approximately midway from a wall of saidcylindrical portion to a vertical central axis of said hopper. 13.Apparatus according to claim 10 including driven brush means forbrushing powder from the lower surface of said upper membrane and theupper surface of said lower membrane.
 14. Apparatus according to claim13 wherein said brush means comprise a pair of brushes mounted on acommon vertical drive shaft, such brush including a horizontal bristleholder and bristles mounted therein.
 15. Apparatus according to claim 10including gas pressure regulating means for controlling gas pressure atsaid inlet and gas pressure at said outlet.
 16. Apparatus according toclaim 10 including heating means for said chamber.
 17. Apparatus forapplying a coating of tungsten carbide to a roto-gravure cylinder in therange of from 15 to 35 microns thickness, said apparatus comprising:aplasma flame spray gun for emitting a spray of molten tungsten carbide,and means for feeding tungsten carbide powder particles of from 1 to 8microns diameter to said spray gun at a substantially uniform density,said feeding means comprising:a chamber-defining hopper including anupper cylindrical portion, a lower conical portion, an inlet at a lowerend of said conical portion for delivering pressurized air to saidchamber, an air outlet disposed at an upper end of said cylindricalportion for exhausting air from said chamber, and means for admittingtungsten carbide powder into said chamber, a lower porous membranedisposed across said conical portion above said air inlet, an upperporous membrane disposed across said cylindrical portion below said airoutlet,said upper and lower porous membranes being pervious to air andimpervious to said powder, said lower membrane disposed below at leastpart of said conical portion such that gas passing through said lowermembrane enters said conical portion and is caused to swirl, powderdischarge opening means opening into said cylindrical portion betweenupper and lower ends of said cylindrical portion and between said porousmembranes, means for supplying pressurized air to said air inlet forsuspending tungsten carbide particles within said chamber between saidupper and lower membranes such that suspended particles from 1 to 8microns diameter are suspended at a substantially uniform densityadjacent said powder discharge means and are conducted therethrough tosaid plasma flame spray gun, pressure control means for controlling airpressure at said air inlet and air outlet, upper and lower rotary brushmeans engageable with a top surface of said lower membrane a bottomsurface of said upper porous membrane, respectively, and means forrotating said upper and lower brush means about a vertical axis to brushparticles from said top and bottom surfaces during passage of airthrough said chamber.
 18. A method for applying a coating of tungstencarbide to a roto-gravure cylinder comprising the steps of:suspendingtungsten carbide powder particles in a gas flow within a hopper suchthat particles having a diameter of from 1 to 8 microns are suspended ata substantially uniform density adjacent a powder discharge of saidhopper, conducting said suspended particles from said discharge to aplasma flame spray torch, and melting said particles in said torch andspraying the melt agains the surface of a roto-gravure cylinder to forma tungsten carbide coating of from 15 to 35 microns thickness thereon.19. A method according to claim 18 wherein said entraining stepcomprises the steps of providing said particles in said hopper betweenupper and lower porous membranes, introducing gas into said hopperbeneath said lower membrane and exhausting said gas from said hopperabove said upper membrane, whereby said gas suspends said powderparticles between said membranes.
 20. A method according to claim 19including the step of brushing powder particles from a lower surface ofsaid upper membrane and from an upper surface of said lower membrane assaid particles are suspended.
 21. A method according to claim 18 whereinsaid molten spray is applied to a roto-gravure printing cylinder.
 22. Amethod according to claim 18, wherein said cylinder comprises a coppercylindrical surface, and wherein prior to said melting and sprayingstep, a film of copper oxide is formed on essentially the entire coppersurface, and said molten spray being applied to said copper oxide film.23. A method of coating a copper surface of a roto-gravure cylinder witha wear resistant substance comprising the steps of:cleaning thecylindrical copper surface of said cylinder, forming a film of copperoxide on essentially the entire cylindrical copper surface of saidcylinder, and spraying said wear-resistant substance in molten form ontosaid copper oxide film to form said coating.
 24. A method according toclaim 23 wherein said wear resistant substance is tungsten carbide. 25.A method according to claim 24 wherein said coating is applied to athickness of from 15 to 35 microns.
 26. A method according to claim 23wherein said copper oxide film is preheated prior to said spraying step.27. A method according to claim 26 wherein said copper oxide film ispreheated to a temperature in the range of from 180° F. to 220° F. 28.Method according to claim 23 wherein said forming step comprisesexposing said cylindrical surface to air.
 29. Method according to claim23 wherein said forming step comprises subjecting said cylindricalsurface to heat.
 30. Method according to claim 24 including the step ofentraining tungsten carbide powder particles in a gas stream within ahopper such that particles having a diameter of from 1 to 8 microns aresuspended at a substantially uniform density adjacent a powder dischargeof said hopper, and conducting said suspended particles from saiddischarge to a flame spray gun.
 31. Apparatus according to claim 10,wherein said discharge opening means faces substantially radially. 32.Apparatus according to claim 17, wherein said discharge opening meansfaces substantially radially. .Iadd.
 33. A method for applying a coatingof wear-resistant material to a roto-gravure cylinder comprising thesteps of:suspending powder particles of said wear-resistant material ina gas flow within a hopper such that particles having a diameter of from1 to 8 microns are suspended at a substantially uniform density adjacenta powder discharge of said hopper, conducting said suspended particlesfrom said discharge to a plasma flame spray torch, and melting saidparticles in said torch and spraying the melt against the surface of theworkpiece to form a wear-resistant coating of from 15 to 35 micronsthickness thereon..Iaddend.